1. Field of the Invention
This invention pertains to the measurement, in situ, of the tension in a high tensile strength bolt.
2. The Prior Art
In the many applications for high tensile strength bolts, the maintenance of proper bolt tension is a critical parameter. If there is insufficient bolt tension, then the joint secured by the bolt will not be as strong as desired. On the other hand, if the bolt is overstressed, a potential failure is built into the joint.
Following initial installation of a high tension bolt, bolt tension may decrease over a period of time due, for example, to vibration. In applications such as bridges, nuclear reactors, etc., it is important to maintain proper bolt tension throughout the life of the installation. Accordingly, it is common practice in such installations to periodically monitor bolt tension in situ. In one common practice, a torque wrench is secured to the bolt head and tightened to the point of impending motion. The reading on the torque wrench at the point of impending motion is taken as an indication of bolt tension. One problem with this approach is that because the torque reading is taken at the point of impending motion, the procedure may result in some slight additional tightening of the bolt. This can result in over-tightening and possible damage, especially when it is considered that this procedure may be performed on each bolt several times over the life of the installation. Another problem with this procedure is that it does not measure actual tension. Rather, it measures torque, and it is well known that forces other than tension contribute to the torque reading. An extreme example occurs when a bolt head is rusted in place. A high torque reading would be shown by the torque wrench, reflecting the torque required to overcome the rust bonding the bolt head to the joint. This high torque reading would then be interpreted as indicating a correspondingly high bolt tension when, in fact, the actual bolt tension might be much lower or non-existent.
Most of the available technology for monitoring bolt tension is concerned with preloading the bolt upon installation. For example, pulse-echo and standing wave measurement ultrasonic techniques are employed for measuring bolt length, which can be directly correlated to bolt tension. While such techniques can be employed to periodically monitor bolt tension following installation, it will be apparent that such techniques are expensive and not suited to most applications. Computer controlled torque systems are also in use for preloading bolts upon installation, but do not provide a simple and effective approach to monitoring bolt tension in situ following installation. These tools and their applications are discussed generally in Electronic Fastener Monitoring: Ensuring The Grip, Assembly Engineering, April, 1984, p. 31; Avoiding Failure In Bolted Joints, Machine Design, May 20, 1982, p. 79; Tools For Reliable Fastening, Assembly Engineering, February, 1982, p. 26; and Fastener Tension Control--What It's All About, Assembly Engineering, November, 1976, p. 22.
Various types of load indicating washers are also available. These washers deform upon installation to indicate proper preload. However, they do not incorporate a mechanism for indicating actual bolt tension following installation. Reference is made, for example, to preload indicating washers marketed by SPS Technologies, Jenkintown, Pa. which employ an arrangement consisting of two concentric steel rings sandwiched between two close tolerance washers. This arrangement relies upon elastic deformation of one sandwiched washer to bind the other sandwiched washer against rotation thereby indicating that the desired preload has been achieved. Another exemplary preload indicating washer is marketed by J&M Turner, Inc., Bristol, Pa. under the designation DTI. That washer includes a plurality of axially extending humps. Compression of the humps upon tightening indicates that the proper preload has been achieved. These washers also do not provide any means for monitoring bolt tension following installation. In addition, because of the intervening humps, the washer does not present a full bearing surface to the bolt head. Not all jurisdictions allow their use, and they are easily tampered with by flattening the humps, as with a hammer, prior to use.
In my issued U.S. Pat. No. 4,303,001, in one embodiment disclosed in FIGS. 8-9, I disclose a washer including a radial slot or groove having a strip of resilient material disposed therein. One end of the strip seats under the bolt head and is subjected to compression as the bolt head is tightened. The spring force is selected such that the other end of the strip deflects upward when the proper bolt tension has been reached. It is also pointed out in the patent that if the bolt loosens, the strip will return to its flat position in the groove, thereby visually indicating a drop in bolt tension. However, this arrangement does not include any means for determining actual bolt tension following installation. Moreover, in practice, it has been found that the amount of deflection of the strip is quite small, and it is consequently difficult to visually determine, upon subsequent inspection, that the strip has returned to its flattened position.
Bolts and nuts have also been proposed which incorporate an element which breaks or separates from the main body of the bolt or nut when the proper preload has been achieved. Reference is made, for example, to the Automatic Tension Control Bolts presently manufactured in Japan by a few firms, such as Sumikin Precision Forge Inc., and marketed by several American importers and distributors, and Guard-Nut offered by Guard-Nut of Sonoma, Calif. Again, however, these devices do not incorporate any means for monitoring bolt tension following installation. Some of these devices are also easily tampered with, and their use is not sanctioned in all jurisdictions.
Special bolts have also been proposed for preloading and, in some instances, providing some indication of bolt tension following installation. One example is RotaBolt offered by Peterson Industries, Inc., Carrollton, Tex. RotaBolt employs a rotatable wheel anchored to a rod member disposed in and anchored to the bottom of an axial bore in the bolt, the wheel is free to rotate until the bolt is tensioned to a specifically designed load. Apart from being expensive, the RotaBolt does not provide a mechanism for measuring the exact load on the bolt at any given time.
Another example is a bolt incorporating a dye-filled chambe. In response to a stress induced crack, the chamber seal is broken whereupon the dye escapes to the surface thereby providing a visual indication of the crack. Again, however, this bolt does not provide a mechanism for measuring actual bolt tension at any given time. An example of this type of bolt is described in the aforementioned Avoiding Failure In Bolted Joints, Assembly Engineering, April, 1984,
80.
Hydraulic stud tensioners are also employed to preload bolts. Basically, these arrangements hydraulically elongate the bolt to achieve the desired preload, whereupon a nut or other fastener is secured to the bolt for retaining the desired elongation. However, following installation, there is no simple way for monitoring the tension in such bolts. See, for example, the aforementioned Fastener Tension Control--What It's All About, Assembly Engineering, November, 1976, p. 30.
For extremely critical applications, bolts with built-in strain gage wires connected to monitoring stations may be employed. Such an arrangement allows monitoring of bolt length, and hence tension, both during and after installation. Such arrangements are, however, expensive and hence impractical for most applications. In some such arrangements pressure transducers attached to the bolts are wired to the monitoring stations. Alternatively, the transducers are fabricated in washer form. Apart from the expense of incorporating a washer at each joint, such transducers are temperature sensitive and typically have an upper limitation of 150 degrees Fahrenheit, which may be exceeded in some applications.
It is accordingly an object of the present invention to provide a relatively simple and inexpensive arrangement for periodically monitoring bolt tension following installation.
It is a further object of the present invention to provide such an arrangement for periodically monitoring bolt tension which further includes means for visually indicating proper preload of the bolt upon installation.
It is yet a further object of the present invention that the means for preloading the bolt upon installation and for periodically monitoring bolt tension following installation is incorporated in a washer, or alternatively, in a nut or bolt head.
It is a further object of the present invention to provide means to determine loading in standard bolts without the need to modify them, such as by drilling axial holes.
It is a further object of the present invention to provide a means to determine loading in bolts without the need to measure changes in distance or length.